A pumping system of this kind is known from the European patent document 0,124,933 A. Further pumping systems are known from the European patent document 1,106,547 A; the German patent documents 39 00 718 A; 1,087,520 A; and the U.S. Pat. documents 2,687,280 and 3,391,963.
The state of the art comprises a pumping system containing two pumps each fitted with a powder aspirating plunger driven by a pneumatic cylinder. The two pumps are driven in opposite directions, therefore one carrying out a suction stroke while the other carries out a pressure stroke. During the suction stroke the associated powder aspirating plunger aspirates powder from a powder supply into its metering chamber. The metered quantity of powder introduced into the metering chamber is expelled by compressed air at the end of the suction stroke out of said metering chamber into a powder feed conduit. Thereupon said plunger will return during a pressure stroke into its initial position from which it will again aspirate powder from said powder supply during a suction stroke. The quantity moved per unit time depends on the frequency of plunger reciprocation. A pumping system of this kind was described in the patent document WO 03/024612 A1 only subsequently to the priority date of the present, new patent application.
Moreover so-called injectors are known wherein, based on the venturi principle, a conveying airflow moves from an outlet nozzle into a collecting nozzle and generates a partial vacuum in the intermediate space, said partial vacuum aspirating coating powder from a powder supply into said conveying airflow. Said injectors incur the drawback relative to the above plunger pumps that the powder particles abrade the collecting nozzle and hence that after some time the efficiency of powder conveyance shall drop. Moving powder in this manner entails a large amount of compressed air per unit time.
The above cited plunger pumps are free of those drawbacks. However they incur another drawback, namely that they move the powder in discontinuous strokes and that more uniform powder conveyance and delivery of larger quantities of powder per unit time require a higher frequency of plunger motion. On the other hand the plunger frequency is limited by the rate at which the valves in the pump's flow paths can be driven. Also care must be paid that the powder particles in the pumps and in their flow paths shall not be squeezed, shall not sinter or adhere to one another and that gaps, recesses and the like shall not arise where powder might accumulate.